1. Field of the Invention
The present invention relates to an optical waveguide element and a method of fabricating an optical waveguide element, and in particular, to an optical waveguide element which can be coupled with an optical fiber at a high coupling rate, and to a method of fabricating the optical waveguide element.
2. Description of the Related Art
Conventionally, glass such as quartz, electro-optical materials and oxide ferroelectrics such as LiNbO3, magneto-optical materials such as Y3Ga5O12, polymers such as PMMA, and GaAs compound semiconductors have been used as materials of planar type optical waveguides. Among these, oxide ferroelectrics are known as exhibiting a particularly good acousto-optical effect and electro-optical effect. However, most acousto-optical elements and electro-optical elements which have been actually fabricated until now utilize LiNbO3 or LiTaO3.
Examples of oxide ferroelectrics are LiNbO3, BaTiO3, PbTiO3, Pb1-xLax(ZryTi1-y)1-x/4O3 (called PZT, PLT, PLZT, depending on the values of x and y), Pb(Mg1/3Nb2/3)O3, KNbO3, LiTaO3, SrxBa1-xNb2O6, PbxBa1-xNb2O6, Bi4Ti3O12, Pb2KNb5O15, K3Li2Nb5O15, and the like. Most of the materials thereamong have better characteristics than LiNbO3. In particular, Pb1-xLax(ZryTi1-y)1-x/4O3 is known as a material having an electro-optical coefficient which is extremely high as compared to that of LiNbO3. The electro-optical coefficient of an LiNbO3 single crystal is 30.9 pm/V, whereas the electro-optical coefficient of a PLZT (8/65/35: x=8%, y=65%, 1-y=35%) ceramic is a large 612 pm/V.
The reason why most elements which are actually fabricated use LiNbO3or LiTaO3, although there are many ferroelectrics having better characteristics than LiNbO3, is as follows. For LiNbO3 and LiTaO3, techniques for growing single crystals and techniques for forming optical waveguides by Ti diffusion into the wafer or proton exchange are established. In contrast, for materials other than LiNbO3 and LiTaO3, a thin film must be formed by epitaxial growth, and a thin film optical waveguide of a quality which can be used in practice cannot be fabricated by conventional vapor phase growth.
In order to overcome the above-described problems, the inventors of the present invention have proposed (in Japanese Patent Application Laid-Open (JP-A) No. 7-78508) a solid phase epitaxial growth technique in which a thin film optical waveguide of a quality which can be used in practice can be fabricated even by an oxide ferroelectric material. However, with this oxide thin film optical waveguide formed by epitaxial growth, a problem arises in that, due to demands for use of a single mode, demands for lowering the driving voltage and the like, there are many cases in which the film thickness cannot be made thin in comparison with the mode field diameter of the optical fiber, and the loss in coupling the optical waveguide with an optical fiber is great.
Conventionally, with semiconductor optical waveguides and quartz waveguides, techniques have been disclosed in which a taper-shaped optical waveguide is provided at a position of connection with an optical fiber, and the coupling loss of the optical waveguide and the optical fiber is reduced (see JP-A Nos. 9-61652, 5-182948, and the like).
However, there is no technique for fabricating a fine pattern which is good for oxide thin film optical waveguides formed by epitaxial growth, and it is difficult to fabricate an optical waveguide in a taper shape. For example, in LiNbO3 single crystal wafers or the like, a method of fabricating a three-dimensional (channel) optical waveguide and grating, to which Ti scattering and proton exchange techniques are applied, is disclosed in xe2x80x9cHikari Shuuseki Kairoxe2x80x9d (xe2x80x9cOptical Integrated Circuitsxe2x80x9d), authored by Nishihara, Haruna, and Suhara, Ohmsha (1993), pp. 195-230. However, for other materials, and for Pb1-xLax(ZryTi1xe2x88x92y)1-x/4O3 in particular, methods of scattering other elements therein or ion exchange are unknown. Further, for quartz optical waveguides and the like, a method of fabricating a channel optical waveguide and the like by reactive ion etching is disclosed in Kawachi, xe2x80x9cNTT RandDxe2x80x9d, 43 (1994) 1273, and the like. However, it is difficult to carry out selective etching without causing surface roughening which is a cause of scattering loss at a single crystal type epitaxial ferroelectric thin film optical waveguide, and without causing damage to the substrate or the like which is an oxide of the same type as the thin film optical waveguide. Thus, there are no reported examples of a channel optical waveguide having little loss being fabricated as an epitaxial ferroelectric thin film optical waveguide.
Further, when making an oxide thin film optical waveguide, which is formed by epitaxial growth, into a taper shape, there is the problem that it is difficult to prevent the waveguide mode from becoming a multimode.
The present invention was developed in order to overcome the above-described problems of the prior art, and an object of the present invention is to provide an optical waveguide element which can be coupled with an optical fiber at a high coupling rate. Further, another object of the present invention is to provide a method of fabricating an optical waveguide element which can precisely fabricate an optical waveguide element which can be coupled with an optical fiber at a high coupling rate.
In order to achieve the above object, an optical waveguide element of the present invention comprises: an optical waveguide layer having a ridge type channel optical waveguide; and a cladding layer provided above at least one of a light entering end portion and a light exiting end portion of the channel optical waveguide of a surface of the optical waveguide layer, the cladding layer having substantially the same width as the channel optical waveguide, and having a refractive index which is smaller than a refractive index of the optical waveguide layer, and having a configuration in which a thickness of the cladding layer increases in a tapered manner toward an end surface.
A method of fabricating an optical waveguide element of the present invention comprises the steps of: (a) forming, on a surface of an optical waveguide layer having a ridge type channel optical waveguide and formed by epitaxial growth, an amorphous thin film whose refractive index after epitaxial growth is smaller than a refractive index of the optical waveguide layer; (b) reshaping the amorphous thin film such that a taper-shaped portion, which has substantially the same width as a width of a channel optical waveguide and has a thickness which increases toward an end surface, remains above at least one of a light entering end portion and a light exiting end portion of the channel optical waveguide; and (c) forming a taper type cladding layer by solid phase epitaxially growing the reshaped amorphous thin film by heating the reshaped amorphous thin film.
Another aspect of the method of fabricating an optical waveguide element of the present invention comprises the steps of: (a) forming, by epitaxial growth and on a surface of a slab type optical waveguide layer formed by epitaxial growth, a slab type cladding layer whose refractive index is smaller than a refractive index of the optical waveguide layer; (b) forming a taper type cladding layer by reshaping the slab type cladding layer such that a taper-shaped portion, which has substantially the same width as a width of a channel optical waveguide and has a thickness which increases toward an end surface, remains above at least one of a light entering end portion and a light exiting end portion at which the channel optical waveguide is to be formed; and (c) forming a ridge type channel optical waveguide by reshaping the slab type optical waveguide layer into a predetermined channel pattern.
In accordance with the optical waveguide element of the present invention, the mode field diameter of the optical waveguide can be enlarged in the direction orthogonal to the substrate surface, and the coupling loss between an optical fiber and the optical waveguide element can be reduced. In particular, because the width of the cladding layer is substantially the same as the width of the channel optical waveguide, the strength of the light confinement in the widthwise direction does not vary drastically at the time when the mode field diameter is enlarged. Generation of loss due to mode mismatching is prevented, and coupling loss with an optical fiber can be greatly reduced.
Further, in the optical waveguide element of the present invention, the thickness of the cladding layer increases in a tapered manner toward the end surface. Thus, the mode field diameter can be increased gradually, and the light propagation loss within the optical waveguide can be reduced.
In a case in which the optical waveguide layer and the cladding layer of the optical waveguide element of the present invention are formed by carrying out patterning while in the state of an amorphous thin film and then solid phase epitaxially growing the patterned amorphous thin film, there is the advantage that it is possible to obtain edges, side walls and surfaces which are extremely smooth and whose light loss due to scattering is small. Further, in a case in which the optical waveguide layer and the cladding layer are formed by patterning thin films which have been solid phase epitaxially grown, there are the advantages that it is possible to obtain an optical waveguide layer and a cladding layer having excellent crystallinity, and that a channel optical waveguide can be formed precisely.